A message still in your Outbox hasn't been 'picked up' by the account you addressed it to, yet. The main difference is that you can still delete or edit it before delivery.

A delivered message hasn't necessarily been read yet, only delivered to the account you've addressed it to when the owner logs in (or maybe when the owner clicks to check new messages, which still isn't the same as reading any new messages). But after that point you can't stop it from being delivered as-is.